Chemical mixing and structural phase transitions; the plateau effect and oscillatory zoning near surfaces and interfaces

The effect of chemical mixing on structural phase transitions is explored. For small concentrations of a solute in the solvent phase, the phase transition T changes non-linearly with increasing solute concentration c. A rather typical T (sub c) vs c curve shows a weak c-dependence of T (sub c) for c up to c (sub p) and a much stronger, often linear, c-dependence for c > c (sub p) . The latter regime represents the effect of classic chemical mixing, whereas the first regime is referred to as the plateau regime. Chemical potentials depend strongly on the structural state of the solvent and, thus, on the order parameter of the solvent phase. This order parameter shows characteristic relaxations near surfaces and interfaces, namely exponential decays or oscillatory patterns; solute particles can decorate these patterns leading to oscillatory zoning inside the mineral without any extrinsic exchange reactions.